1. Field of the Invention
This invention relates to a method of and apparatus for disposal and treatment of waste.
2. Description of the Related Art
This invention has particular but not exclusive application to the production of compost from domestic organic waste using a composting toilet, and for illustrative purposes, reference will be made to such an application. However, it is to be understood that this invention could be used in other applications such as production of compost and/or disposal of waste in industrial, commercial and other situations. Compost material may be defined as waste material that has become stable through biological action. Composting is performed in a compost bed which may be defined as a bed of composting and composted organic solid wastes, the composted material being completely decomposed organic matter and the composting material being material including raw organic wastes and decomposing organic material.
Organic wastes can be classified into two general forms. Solid wastes such as paper, cellulose based products, food wastes and yard wastes, can be defined as Organic Municipal Solid Wastes (OMSW). Liquid organic wastes, such as sullage water and sewage water, can be defined as Wastewater. Unless the context dictates otherwise, wastewater hereinafter may be taken to have this meaning.
Current waste treatment technologies are, with the exception of large scale methane digestion, based on separate treatment processes for the high solids OMSW and low solids wastewater. Typically the solids component of wastewater is separated from the water during anaerobic fermentation by sedimentation of the solids as a sludge which is then de-watered and is commonly composted with an organic bulking agent such as wood waste, or co-composted with OMSW.
In the past land disposal of the sludge mixed with lime to partially sanitise was common. Supernatant liquid formed as a result of the settling out of the solids from the wastewater in the primary treatment process is referred to as primary effluent which, without further, secondary treatment, is not safe to discharge into the general environment. A process to further treat the wastewater relying on aeration of the primary treated effluent in the presence of aerobic organisms, which utilise degradable organic matter and bacteria in the effluent; as food sources, and so purify the wastewater and reduce its environmental hazard has been developed and commercialised.
Further settling of sludge occurs in a secondary treatment vessel, designed to maximise the biological breakdown of suspended and dissolved organic matter, by a fixed or mobile biofilm of aerobic microbes. The primary effluent entering the secondary treatment vessel is aerated by blowing air over or through a large liquid contact surface to assist in and support the growth of the aerobic biofilm. After secondary treatment, the effluent is usually disinfected and discharged.
Such conventional methods of wastewater treatment have several disadvantages. The usual scale of treatment is at the municipal scale, where the process is reliant on high energy inputs for pumping, aeration, and construction of specialised apparatus for process control, monitoring, and materials handling, management and staffing inputs are high and a costly and complicated transport and treatment infrastructure is essential.
The municipal treatment of OMSW is similarly expensive and troublesome. Regulations are being formulated in many countries which prohibit OMSW disposal in landfill. Composting, though the most favoured method of treatment, is not without problems. Odour problems have caused some compost plant operators to enclose the operation so that odorous air may be filtered through a bio-filtration apparatus and/or odour scrubbers prior to discharge of the spent air.
This problem, coupled with high levels of contaminants if industrial wastes are combined with domestic wastewater, have caused some sewage plant operators to opt for incineration as a more convenient path despite the large amounts of energy required for drying before combustion.
Co-composting can often reduce the amount of municipal solid waste going to landfill by as much as 60%, representing a significant saving. Where co-composting of sewage sludge and OMSW is practiced, it is still expensive, due to the need to collect, sort, transport and process the organic portion before composting.
A large proportion of the cost associated with sewage sludge when centralised is attributed to establishing and maintaining systems for the collection and transport of the wastes, and transport of sewage sludge is the major cost involved. Wherever sewage sludge is incorporated, there can be consumer resistance to the product because of perceived health risks from biological and/or industrial contaminants.
On-site household wastewater treatment systems can, with proper maintenance, recycle water for gardens, whilst various household composting devices are being employed to recycle nutrients and humus back into the soil. The risk of spreading waterborne diseases should be lessened, but only where contact with wastewater is restricted to those likely to have developed immunity to any diseases contained in the wastewater.
On a household scale, on-site wastewater treatment is not preferred over centralised treatment, partly because of its inconsistent performance, and the requirement for ongoing monitoring of each installation. The anaerobic digestion stage is prone to the formation of bacterial scums and crusting which can lead to odour and performance problems.
Present household wastewater treatment systems, need indefinite, specialised and costly management intervention, are sensitive to common household chemicals and require ongoing costly maintenance. At the household scale chlorine based disinfection is typically carried out on the liquid effluent, using hazardous chemicals and resulting in the production of toxic disinfection by-products. Additionally, on-site composting has the inconvenience the potential for vermin and disease vectors to gain access to such composting apparatus and require sorting and user management intervention to produce good compost.